Compositions which contain an alkali-soluble resin binder such as a novolak resin and a naphthoquinonediazide compound as a photosensitive material have been generally used as a positive photoresist composition.
As the binder, a novolak resin is particularly useful, because it is soluble in an aqueous alkali solution without swelling and offers high resistance, particularly to plasma etching when an image formed is used as a mask for etching. The naphthoquinonediazide compound, which is used as the photosensitive material, itself acts as a dissolution inhibitor to lower alkali solubility of the novolak resin. However, when the naphthoquinone-diazide compound is decomposed by irradiation with light to form an alkali-soluble substance, the alkali solubility of the novolak resin is increased. Due to such a great change in nature which depends on light, the naphthoquinonediazide compound is particularly useful as the photosensitive material for the positive photoresists.
From this standpoint, a number of positive photoresist compositions containing a novolak resin and a naphthoquinonediazide photosensitive material have hitherto been developed and put to practical use. Particularly, such photoresists are making remarkable progress toward high resolution to achieve striking results with regard to processing of line width up to submicron.
To obtain higher resolution and reproduce an image having a better pattern shape, use of a photoresist having a higher contrast (.gamma. value) has been considered advantageous and the development of such photoresist compositions has been carried out. There are a great number of publications in which such techniques are disclosed. With respect to the novolak resins, one of the main components of the positive photoresist composition, a number of patent applications disclose specific compositions of monomers, distributions of molecular weight, and processes for the preparation of the novolak resins, providing an improvement to some extent. Also with respect to the photosensitive material, the other main component, a number of compounds having structures which are recognized as effective for obtaining a higher contrast have been disclosed. When positive photoresists are designed with the aid of these techniques, it is possible to develop a resist having extra high resolving power so that a pattern having a dimension similar to wavelength of light can be resolved.
On the other hand, the integrated circuits increasingly need higher integrity thereof. In a process for the manufacture of semiconductor substrates such as extra large scale integration, it comes to be required to process an ultra fine pattern consisting of a line width of 0.5 .mu.m or less. Such uses require the photoresists to have high resolution and a broad latitude of development to constantly ensure a definite line width. In addition, to prevent processing defects of the circuits, the photoresists are required to leave no photoresist residue on the patterns after development.
Further, particularly in the formation of the ultra fine pattern consisting of the line width of 0.5 .mu.m or less, it has been found that, even if the photoresists have good resolution in a certain thickness, a phenomenon that the slightest change in thickness causes the resolution to be deteriorated (hereinafter referred to as "dependence on film thickness") is observed. Surprisingly, even when the thickness changes only by a few hundredths .mu.m, the resolution considerably changes, and any one of typical positive photoresists which are now commercially available has also been found to exhibit this tendency more or less. Specifically, when the thickness of a resist film before exposure changes from a certain film thickness in the range of .lambda./4 n (.lambda. represents the wavelength of an exposing ray; and n represents the refractive index of a resist film at this wavelength), the resolution also changes depending upon the change in film thickness.
The presence of this problem, the dependence on film thickness, is pointed out, for example, in SPIE Proceedings, Vol. 1925, p. 626 (1993), and it is described therein that a multiple reflective effect of a ray inside a resist film causes the problem.
There are many cases where an increase of the contrast of a resist to obtain high resolution and a pattern having a rectangular shape in its cross section causes the dependence on film thickness to increase. In fact, on processing a semiconductor substrate, a pattern is compelled to be formed on a resist film which has subtly different thickness all over the surface, because of unevenness on the surface of the substrate and irregularity in film thickness caused by coating. Therefore, the dependence on film thickness becomes an obstacle to a positive photoresist in which a ultra fine processing must be conducted to an almost full extent of limitation of the resolution.
Further, requirements for particles of positive photoresists are more and more increasing with the promotion of integration in semiconductors. In the semiconductors, particles having sizes of a tenth or more of the least line width of a device have the effect on the yield, as generally indicated by the 1/10 rule (Ultra Clean Technology, Vol. 3, No. 1, p. 79 (1991), etc.).
To decrease these particles, use of an ultra fine filter having a pore diameter of 0.1 .mu.m or 0.05 .mu.m during the manufacturing step of resists is devised. This is, in fact, useful for the reduction of such particles on manufacturing the resists.
However, in spite of the reduction of particles at the manufacturing step of a resist, the number of particles in a resist often increases with a lapse of time. The increase in particles with time is almost caused by 1,2-quinonediazides which are used as the photosensitive material. Various attempts have been hitherto made to prevent the number of particles to increase with time.
Examples of such attempts include a method of using a photosensitive material in which part of hydroxy groups of a polyhydroxy compound undergo acylation or sulfonylation (JP-A-62-178562; the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a method of using a mixture of 1,2-naphthoquinonediazide-4-and -5-sulfonic esters (JP-A-62-284354), a method of using a heat-modified 1,2-naphthoquinonediazide as a photosensitive material (JP-A-63-113451), a method of decreasing a catalyst remaining in a photosensitive material (JP-A-63-236030), a method of synthesizing a photosensitive material in the presence of an anion exchange resin (JP-A-63-236031), and a method of mixing a solvent which has excellent solubility for photosensitive materials (JP-A-61-260239 and JP-A-1-293340).
To improve the resolution, a large number of 1,2-naphthoquinonediazides of polyhydroxy compounds having particular structures have hitherto been proposed. Examples thereof are disclosed in JP-A-57-63526, JP-A-60-163043, JP-A-62-10645, JP-A-62-10646, JP-A-62-150245, JP-A-63-220139, JP-A-64-76047, JP-A-1-189644, JP-A-2-285351, JP-A-2-296248, JP-A-2-296249, JP-A-3-48249, JP-A-3-48250, JP-A-3-158856, JP-A-3-228057, JP-W-4-502519 (The term "JP-W" as used herein means an unexamined published international patent application), U.S. Pat. Nos. 4,957,846, 4,992,356, 5,151,340, and 5,178,986, European Patent 530,148, and so forth. However, use of these photosensitive materials has been still insufficient with regard to decrease in the dependence on film thickness.
On the other hand, resists having a high contrast and high resolution are obtained by use of photosensitive materials containing hydroxyl groups in molecule as described in JP-B-37-18015, JP-A-58-150948, JP-A-2-19846, JP-A-2-103543, JP-A-3-228057, JP-A-5-323597, JP-A-6-148878, JP-A-6-167805, JP-A-6-202321, U.S. Pat. Nos. 3,061,430, 3,130,047, 3,130,048, 3,130,049, 3,102,809, 3,184,310, 3,188,210, and 3,180,733, West German Patent No. 938,233, SPIE Proceedings, Vol. 631, p. 210, ibid., Vol. 1672, p. 231 (1992), ibid, vol. 1672, p. 262 (1992), and ibid., Vol. 1925, p. 227 (1993).
However, the photosensitive materials containing hydroxyl groups in molecule which are described in the above-mentioned specifications are not sufficiently satisfactory, because of further increased requisitions accompanied by the promotion of integration of the semiconductors.